Most sudden heart deaths are due to ventricular fibrillation in patients both with and without coronary disease. Ventricular fibrillation consists of chaotic, asynchronous and fractional activity of the ventricles. In a heart which has started the ventricular fibrillation process, all of the cells contract independently and not synchronously, with the final result that the pumping function of the heart is lost and circulatory arrest occurs; without intervention, the patient dies.
The only way of intervening is by electric heart defibrillation. This method, which was implemented successfully as early as 1908, came back into clinical practice around the 1940s and has been used increasingly since then. External defibrillation is achieved by applying to the patient's chest two plates by means of which an electric shock is transmitted. In recent years, implantable electric defibrillators have been designed and produced and these apply the electric shock directly to the heart wall, the shock being delivered automatically as soon as ventricular fibrillation is recognized by the circuits.
It should also be noted that malignant ventricular tachycardia (MVT), which is usually a precursor of ventricular fibrillation, can also be treated by electric cardioversion. A stimulation system with anti-tachycardia programs (with burst and premature extra-stimulation capabilities and the like) which are used as a first approach for the cardioversion of MVT is fitted, together with the defibrillation system, in the same device. In serious cases which are insensitive to anti-arrhythmic stimulation, when a certain number of attempts with this program have been found ineffective, the system can deliver an electric shock which has a greater probability of interrupting the MVT but which usually has less energy than for ventricular fibrillation.
The implantation of these devices (ICDs--implantable cardioverter defibrillators) started in 1980 and, since the 1990s, has increased notably because of the considerable technological progress and the increased ease of implantation due, in particular, to the use of catheter-electrodes which are introduced by a peripheral venous route in the same manner as for the implantation of pacemakers.
The implantation of ICDs is currently the only safe means of ensuring the survival of patients affected by these arrhythmias which are otherwise fatal.
There are, however, considerable problems, including:
a) the harmful nature of the electric shock which, with the energy of about 30 Joules currently used, damages the mitochondrial structures of the cells, and
b) the fact that the life of the patient wearing the ICD is rendered traumatic by the loss of consciousness which occurs in the presence of ventricular fibrillation and hence of defibrillation, even though this saves the patient's life.
In fact, the shock is delivered about 10 seconds after ventricular fibrillation is recognized; this leads to circulatory arrest with loss of flow of oxygenated blood to the brain so that, after 5-6 seconds, the patient loses consciousness, falling to the ground if he is standing up.
The delay in the delivery of the shock is necessary in order to confirm the diagnosis of ventricular fibrillation and to charge the capacitor which serves to store the energy for the delivery of the shock.
The delay is also necessary in order to deliver the shock when the patient is unconscious so that he does not feel the pain of the discharge. In some cases of younger patients who are still conscious when the discharge is delivered, the sensation of pain is in fact so strong and distressing that some patients have asked for the ICD to be removed.
The situation in which it is necessary to interrupt MVT by means of the shock should also be considered; in fact the delivery of the shock takes place when the patient is fully conscious since, although MVT is disabling, it does not cause loss of consciousness. In these cases, the pain complained of by the patient which, amongst other things, is sudden, is very great, although it is not of long duration.
In any case, even in patients who do not feel the shock, their existence becomes so traumatic with the continual fear and expectation of crises accompanied by loss of consciousness that they sometimes prefer the risk of death.
A condition which is similar to ventricular fibrillation as a physiological phenomenon, although it does not involve an immediate danger of death of the patient will now also be considered.
Atrial fibrillation (AF) is an arrhythmia which causes disappearance of the atrial contractions which are replaced by fibrillation, that is, by uncoordinated activity which nullifies the pumping effect of normal contraction. It is compatible with life since blood circulation is maintained, although with a reduction of the cardiac blood flow.
However, atrial fibrillation causes stagnation of the blood in the atrium which favors the formation of a thromboembolism which, in time, puts the patient's life at risk. Moreover, the irregularity of the ventricular response may set off dangerous ventricular tachycardia. This arrhythmia can be treated pharmacologically but insensitivity to the drugs is often encountered.
Another possibility is electric cardioversion which consists of the application of an electric shock similar to ventricular defibrillation but with lower discharge energy. A couple of years ago, the implanted atrial defibrillator technique was proposed and implemented in order to deliver a shock directly to the appropriate locations of the heart cavity at the onset of the arrhythmia. For this treatment, the need to attenuate or cancel out the pain caused by the discharge which, in this case, is applied to conscious patients who have difficulty in tolerating it, becomes fundamental.
It can be seen from the foregoing description that an ability to prevent the pain signals from being perceived by the patient is very important.